[libvirt] [PATCH] docs: correct setmem text plus expand setmaxmem text

[Justin Clift]

On 20/01/2011, at 2:05 AM, Daniel P. Berrange wrote:
> On Wed, Jan 19, 2011 at 07:54:52AM -0700, Eric Blake wrote:
>> On 01/18/2011 05:28 PM, Justin Clift wrote:
>>> This completes the man page updates required for BZ # 622534:
>>> 
>>>  https://bugzilla.redhat.com/show_bug.cgi?id=622534
>>> =item B<setmaxmem> I<domain-id> B<kilobytes>
>>> 
>>> -Change the maximum memory allocation limit in the guest domain. This should
>>> -not change the current memory use. The memory limit is specified in
>>> -kilobytes.
>>> +Change the maximum memory allocation limit for an active guest domain.
>> 
>> I'm a bit confused here.  Doesn't maximum memory allocation only take
>> effect at domain boot?  So it seems like something you can't change at
>> runtime, and if a change is made to an active domain, it only affects
>> the configured version and the next boot of the domain rather than
>> having an immediate effect.  But I haven't played closely with
>> setmaxmem, so I could be wrong.
> 
> In theory we could change maxmem at runtime, if the guest truely
> supported memory hotplug, instead of simply ballooning. No hypervisor
> or guest in existance really does that yet. So 'max mem' should be
> considered a boot time only tunable at this time.

The virsh "setmaxmem" command we have at the moment seems to be for
changing the max value of an active domain, and doesn't yet appear to
have any capability for changing the persistent XML config.

Trying it out on a F14 guest, running on RHEL 6.0, confirms this isn't a supported
operation on running guests with the RHEL 6.0 Qemu/KVM hypervisor:

  # setmaxmem Fedora_14_x64 786432
  error: Unable to change MaxMemorySize
  error: this function is not supported by the connection driver: virDomainSetMaxMemory

Went looking at the commit that added this command to virsh:

  commit 9425a3e617aff567b2eaa1713cda4790aab62b8b
  Author: Daniel P. Berrange <berrange at redhat.com>
  Date:   Wed Aug 16 17:30:33 2006 +0000

      Added setvcpus, setmem, setmaxme commands to virsh

Wasn't anything super informative in there, but the commit immediately before
it looks like it might have relevance:

  commit 5c2831b30b3cb0a8f422006a87f0423c74d26707
  Author: Daniel P. Berrange <berrange at redhat.com>
  Date:   Wed Aug 16 16:36:39 2006 +0000

      Hook up more test driver methods. Allow HV config to be loaded from extenral XML definition

Is there any chance that the "setmaxmem" command as presently implemented, is
really only suitable for use by a test driver of some sort?

Regards and best wishes,

Justin Clift